Abstract
1. Growth of Ultralong ZnO Nanowires on Silicon Substrates by Vapor Transport and Their Use as Recyclable Photocatalysts We synthesized the growth of ultralong ZnO nanowires on silicon (100) substrates via the gold-catalyzed vapor transport approach. Ample supply of zinc vapor generated through carbothermal reduction of ZnO powder at 917 °C and a suitable amount of oxygen facilitate the rapid growth of nanowires. These ZnO nanowires are extremely long with lengths of 85–100 microns, and exhibit a vertical orientation. The nanowires have largely diameters of 250–400 nm. Crystal structure analysis indicates typical ZnO nanowire growth along the [0001] direction. The band gap of these nanowires was determined to be 3.22 eV. These nanowires show a relatively weak near-band-edge emission peak at 390 nm, and a significant oxygen vacancy-related emission band at 495 nm. Good photocatalytic activity of these nanowires on substrates toward the photodegradation of rhodamine B and 4-chlorophenol was demonstrated. Furthermore, we showed that these nanowires on substrates can serve as effective and convenient recyclable photocatalysts. Only a slight decrease in the photodecomposition rate was observed after 10 cycles of the photocatalysis experiment. The photocatalysts also work well under natural sunlight. 2. Gold-Catalyzed Low-Temperature Growth of Cadmium Oxide Nanowires by Vapor Transport Uniform and ultralong cadmium oxide nanowires were synthesized in high yield on gold-coated silicon substrates using a vapor transport process. Cadmium vapor generated by the carbothermal reduction of CdO powder in a tube furnace heated to 500 °C was carried to the substrate zone by an argon flow with a trace amount of oxygen. The CdO nanowires grew via a vapor-liquid-solid growth mechanism. The diameters of the nanowires are 80 nm, and can reach lengths of 40 microns. Because the nanowire formation was gold particle-catalyzed, patterned nanowire growth on substrates can be achieved. These nanowires grew along the [111] direction and have slight rough surfaces due to the presence of crystalline CdO shells formed via a vapor-solid deposition process. Interesting CdO nanowires with a necklace-like morphology were also observed in a small region of the substrate, where the oxygen supply may be ample to facilitate the lateral growth of rhombohedral crystals over the pre-formed central wires. Electron diffraction and high-resolution TEM results suggest that the rhombohedral crystals should grow epitaxially from the wire stem. The band gap of the CdO nanowires was determined to be ~2.53 eV. These nanowires exhibit a relatively weak emission band centered at ~550 nm. 3. Synthesis of Self-Templated Core-Shell Ga-GaN Nanostructures and GaN Hollow Spheres via Conventional Reflux Method Core-shell Ga-GaN nanostructures were synthesized via a simple reflux method. Gallium chloride was reacted with lithium bis(trimethylsilyl)amide in trioctylamine at 380 □C for 24 hours. The core-shell nanostructures are ~500 nm in diameter with a rough surface. After removal of the gallium core by acid, GaN hollow spheres can be obtained. The GaN hollow spheres are composed by GaN nanoparticles with the shell thickness ~25 nm. PXRD and ED pattern show the GaN hollow spheres are zinc blende structure with hexagonal structure less than 10%. By XPS and IR analysis, the GaN nanoparticles are binding by silica to maintain the hollow structure. The growth of Ga-GaN nanostructures were monitored during the increase of temperature and reaction time. Gallium was generated first at lower reaction temperature and followed by GaN deposited on the metal surface. The size and shell thickness of Ga-GaN nanostructures can be varied by changing the amount of lithium bis(trimethylsilyl)amide and reaction temperatures. Furthermore, In-InN nanocomposites also can be synthesized using the same reflux method with a lower reaction temperature and longer reaction time. The InN nanoparticles are wedge-shaped or triangular with a single crystallinity.